Attach source catalog even for follow only mode (#596)

test_decoding transform relies on source catalog to decode the update message.

Prior to this commit, update while using test_decoding would work only while doing `pgcopydb clone --follow`. This commit enables support for update message decoding
while using test_decoding plugin in `pgcopydb follow` too.

Signed-off-by: Arunprasad Rajkumar <ar.arunprasad@gmail.com>